Alright then Kitsky!

You have three options as I see it at that pricepoint:

1) Patiently search Craigslist for an old '80's chromoly steel bicycle. It doesn't matter what speed it is...
Try to find a bicycle co-op in your area. If there are none (and I don't think there are any), try to find someone who is knowledgeable about bicycles. Take this person (the expert) with you, when you go to buy your prospective bicycle. You must make certain that the welds at the joints are perfect. There should be no rust issues anywhere on the frame. Since you're female, you might just try to find a female frame. However, women ride male frames all the time. If it's a good bike and properly fits, ride it! Try not to pay over $100....You're possibly gonna need something extra for the LBS to get it back up to speed.

2) You could buy this single speed bike online thru Nashbar.com....It's called, The Hounder ~ $200
www.nashbar.com/bikes/Product_10053_10052_504148_-1_202339

* This bike is made of chromoly steel and is very reliable. It will arrive 95% assembled already. You'll just have to place the finishing touches on the assembly, in order for it to be complete.

This link should assist you in the completion of the final assembly:
www.bikesdirect.com/instructionhelp.htm
(Watch the video a whole bunch of times if need be)

3) If you must go to Walmart, don't get the Thruster! Get either the GMC Denali or the Schwinn Varsity. Any Schwinn that costs over $150 OTD, would be marginally borderline begrudgingly reluctantly acceptable. That's only if you're absolutely bankrupt!

Good Luck!

- Slim